Radiation curable coating compositions are used in many industries including, but not limited to fiber optic materials and coatings for various substrates such as concrete, metal, ceramic, glass, plastic, composites and textiles. Common types of radiation curable compositions are those compositions curable by free radical polymerization. In these compositions, the radiation (e.g., UV radiation) is absorbed by the composition to effect curing or polymerization via the generation of free radicals. The curing of the composition is accomplished by photoinitiators, which absorb the UV energy and react to generate free radicals, which in turn react with double bonds in the composition (e.g., acrylate groups) to form new free radicals (i.e., the initiation step). The newly formed free radicals then react with other double bond centers to polymerize or cure (i.e., solidify) the uncured, liquid composition in the propagation step. Eventually, the polymerization reaction is terminated when the free radicals react with other free radicals instead of reacting with other reactive sites to form new free radicals. This step is aptly referred to as the termination step. In view of the foregoing, it is apparent that the choice of photoinitiator is important to the success of a free radical polymerization process.
A review of photoinitiators for UV curing is disclosed in “A Compilation of Photoinitiators Commercially Available for UV Today” by Dr. Kurt Dietliker of Ciba Specialty Chemicals PLC published by SITA Technology Limited (2002), which is incorporated herein by reference in its entirety. In April 2009, Ciba Holding AG was acquired by BASF.
Radiation-curable compositions are extensively used in the optical fiber industry during the production of optical fibers, ribbons and cables. For example, optical glass fibers are routinely coated with at least two radiation-curable coatings immediately after the glass fiber is manufactured in a draw tower so as to preserve the pristine character of the glass fiber and protect it sufficiently such that it can be collected on a round spool. Immediately after a coating is applied to the fiber, the coating is rapidly cured by exposure to radiation (commonly ultraviolet light). The industry demands faster production speeds and therefore, faster curing coating compositions.
Radiation-curable up-jacketing, matrix and bundling materials can further support and protect the individual strands of coated fiber as individual strands are bundled together into optical fiber ribbons, optical fiber cables and associated structures. In addition, radiation-curable inks can be used to color code individual strands of optical fiber. All of these types of optical fiber-related materials are radiation-curable and can serve as coating and/or cabling materials.
Examples of radiation-curable inner primary coatings are disclosed in U.S. Pat. No. 5,336,563 to Coady et al. and of outer primary coatings (e.g., secondary coatings) in U.S. Pat. No. 4,472,019 to Bishop et al. Additional aspects of optical fiber coating technology are disclosed in, for example, U.S. Pat. No. 5,595,820 to Szum; U.S. Pat. No. 5,199,098 to Nolan et al.; U.S. Pat. No. 4,923,915 to Urruti et al.; U.S. Pat. No. 4,720,529 to Kimura et al.; and U.S. Pat. No. 4,474,830 to Taylor et al., each of which is incorporated herein by reference in their entirety.
The following U.S. patent applications, describing and claiming illustrative radiation curable coating compositions, are incorporated by reference in their entirety: U.S. patent application Ser. No. 11/955,935, filed Dec. 13, 2007, published as US 20080226916 on Sep. 19, 2008; U.S. patent application Ser. No. 11/955,838, filed Dec. 13, 2007, published as US 20080241535 on Oct. 23, 2008; U.S. patent application Ser. No. 11/955,547, filed Dec. 13, 2007, published as US 20080226912 on Sep. 19, 2008; U.S. patent application Ser. No. 11/955,614, filed Dec. 13, 2007, published as US 20080226914 on Sep. 19, 2008; U.S. patent application Ser. No. 11/955,604, filed Dec. 13, 2007, published as US 20080226913 on Sep. 19, 2008; U.S. patent application Ser. No. 11/955,721, filed Dec. 13, 2007, published as US 20080233397 on Sep. 25, 2008; U.S. patent application Ser. No. 11/955,525, filed Dec. 13, 2007, published as US 20080226911 on Sep. 19, 2008; U.S. patent application Ser. No. 11/955,628, filed Dec. 13, 2007, published as US 20080226915 on Sep. 19, 2008; and U.S. patent application Ser. No. 11/955,541, filed Dec. 13, 2007, published as US 20080226909 on Sep. 19, 2008.
Radiation-curable coatings are used as coatings for concrete and metal. UV curable concrete coatings are discussed, for example, in the article, “UV Curable Concrete Coatings” by Jo Ann Arceneaux, Ph.D., Cytec Industries Inc., Smyrna, Ga., presented at the Federation of Societies for Coatings Technology, “Coatings for Concrete Conference: “Coating the World of Concrete,” on Feb. 2, 2009 at the Westin Casuarina Las Vegas Hotel in Las Vegas, Nev. and in the article, “Field-Applied, UV-Curable Coatings for Concrete Flooring,” by Peter T. Weissman, published in the January/February/March 2009 RADTECH Report.
UVolve® Instant Floor Coatings (available from DSM), are high performance, instant cure coating systems for concrete floors which have the following features and benefits:                virtually instantly curing ability allows for immediate traffic—even forklift;        one-component system with no mixing, with no pot life constraints or wasted product;        the cured coating protects concrete against damage from dirt, wear and chemicals; and        cured UVolve® Instant Floor Coatings clean easily—especially forklift tire marks.The use of radiation curable coatings for concrete floors means that the facility maintenance costs will be lower due to easy clean. UVolve® instant Floor Coatings have zero VOC, no solvents, and 100% solids. UVolve® Instant Floor Coatings cure to a high gloss, durable finish which exhibits excellent scratch and impact resistance. They are available in both clear and pigmented systems and cure instantly with the use of a UV light machine specifically designed for use with UVolve® Instant Floor Coatings. See: http://www.uvolvecoatings.com/.        
The UVaCorr® Corrosion-Resistant UV Coatings for Tube & Pipe (UVaCorr® products are available from DSM), are high performance, radiation curable coating systems used to improve the corrosion resistance of tube and pipe. The UVaCorr® coatings are available as both clear and colored coatings and are used to protect tube and pipe during storage and transport. The UVaCorr® product line, now certified for use in the Venjakob™ Ven Spray Pipe system (trademark of Venjakob), boast several performance advantages over traditional solvent-based and water-borne tube & pipe coatings, including: instant cure for high-speed processing; 100% solid coatings for higher applied coverage and no VOC's; better salt spray resistance for enhanced performance; and smaller equipment footprint with reduced energy requirements. See: http://www.dsm.com/en_US/html/dsmd/uvention_tube.htm
To maximize cure speed in an ultraviolet light cure, at least one photoinitiator is required (photoinitiator may be omitted in an electron beam cure). Several photoinitiators can be used to achieve a suitable balance of surface and through cure. For further discussion of the use of more than one photoinitiator see U.S. Pat. Nos. 6,438,306 and 7,276,543. When more than one photoinitiator is present in a radiation curable composition of the invention, conventional classes of photoinitiators have been found to be useful.
Mono-acyl phosphine oxide type photoinitiators can be used such as Lucirin TPO (2,4,6-trimethylbenzoyl) diphenyl phosphine oxide, available commercially from BASF, which exhibits relatively fast cure speed. However, use of commercial Lucirin TPO can cause undesired crystallization effects in coating compositions (e.g., during aging), which can result in inclusions and loss of optical clarity (detected under a light microscope).
Certain photoinitiators are known to cause yellowing, particularly during long term aging of cured compositions under photolytic aging conditions (e.g. UV or fluorescent light). Heat may also induce yellowing. Discoloration in general and yellowing in particular is undesirable and has become anathema in the industry. Hence, a photoinitiator which would provide for lack of harmful crystalline effects and fast cure, but would result in yellowing, would not sufficiently meet the most stringent industry demands.
Attempts have been made to use purified Lucirin TPO, but the purification steps are costly. Other phosphine oxide photoinitiators (e.g., CGI 403, Ciba) can show reduced amounts of harmful crystallization effect, but they may also have slower cure speed. Hence, it is desirable to provide photoinitiators which can provide both fast cure speed and good optical clarity.
Other desirable performance properties for radiation curable media include: being a liquid at ordinary temperatures and having a sufficiently low viscosity to be excellently coated; providing good productivity at a high curing rate; having sufficient strength and superior flexibility; exhibiting very little physical change during temperature changes over a wide range; having superior heat resistance and superior resistance to hydrolysis; showing superior long term reliability with little physical changes over time; showing superior resistance to chemicals such as acids and alkalis exhibiting low moisture and water absorption; exhibiting superior light resistance showing the least discoloration over time; and exhibiting high resistance to oils. Moreover, an increased demand for processing speed of cured materials makes it necessary for the coating compositions to cure quickly in a stable manner. Thus, a photo-initiator(s) which decomposes fast must be used for the coating materials to cure quickly.
As of the filing date of the instant application, the art is yet to recognize a photoinitiator which provides an excellent balance of all of these critical properties. For example, a large number of phosphine oxide photoinitiators are disclosed in, for example, U.S. Pat. No. 5,218,009 to Rutsch et al. and U.S. Pat. No. 5,534,559 to Leppard et al. However, these patents do not suggest that any particular species of photoinitiators would solve the above-noted problems and provide an excellent balance of properties.
Japanese Patent Application Laid-open No. 190712/1989 discloses a composition comprising an acyl phosphine oxide as a photo-curable resin composition which realizes high productivity in fast curing. However, this composition is not necessarily cured at a high enough rate to sufficiently increase the productivity of optical fibers while maintaining the characteristics required for an optical fiber coating material.
Another composition comprising a bis-acyl phosphine oxide has been proposed in Japanese Patent Application Laid-open No. 259642/1996 as a photo-curable resin composition which shows high productivity by being cured at a high rate. However, the bis-acyl phosphine oxide containing a long chain aliphatic group disclosed in this Japanese Patent Application has a poor solubility in resin compositions, and hence cannot be dissolved in the resin compositions in an amount sufficient to ensure a high cure rate.
U.S. Pat. Nos. 6,136,880 and 6,359,025 and EP Patent Application EP 0975693 to Snowwhite et al. disclose radiation curable coating compositions for optical fiber comprising solid bis-acylphosphine oxide (BAPO) type photoinitiators.
Bis-acylphosphine oxide (i.e., bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide) (BAPO) is a very potent photoinitiator in radiation curable compositions light-induced polymerization of the ethylenically unsaturated compounds. It has a higher extinction coefficient than acyl phosphine oxides such as TPO or TPO-L and thus typically leads to superb photo speed. However. BAPO is a solid having a low solubility in a variety of monomers and oligomers, which limits its use in some applications.
In an attempt to address the shortcomings of solid BAPO, liquid photoinitiator mixtures of BAPO with bis-acylphosphine (BAP) have been reported. For example, see “Liquid Bis-Acylphosphine Oxide (BAPO) Photoinitiators” by C. C. Chiu from Chitec Technology presented at RADTECH 2010 on Monday, May 24, 2010.
In the Chiu presentation, liquid mixtures of BAPO and BAP (collectively known as “LMBAPO”) are described. Although the liquid mixture of BAPO and BAP photoinitiators (i.e., LMBAPO) purportedly has film-curing properties similar to solid BAPO, LMBAPO suffers from poor chemical stability, which limits its industrial application.
Thus, there remains an unmet need for photoinitiators suitable for radiation curable compositions exhibiting a balance of the critical performance properties, including existing in a liquid state for radiation curable compositions.